Non-contact electrical machine air gap measurement tool

ABSTRACT

A measuring kit for contactless measuring of the air gap distance between a frame mounted pole and core of a rotor includes a capacitance sensor which generates a signal proportional to the measured air gap, a panel meter in communication with the capacitance sensor which interprets the signal and displays the minimum air gap distance, an A/D converter also in communication with the capacitance sensor which converts the signal to a digitized signal, and a control panel which takes the digitized signal from the A/D converter, processes the digitized signal, and then displays the minimum air gap measurement. The control panel shows a graphic of the core and its surrounding poles to track the progress of the testing, and when the testing between the core and one of the surrounding poles is complete, the graphic of the pole tested visually darkens to indicate that portion of the test is complete.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein was made in the performance of officialduties by one or more employees of the Department of the Navy, and thus,the invention herein may be manufactured, used or licensed by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

FIELD OF THE INVENTION

The disclosure relates generally to a non-contact method and tool formeasuring the air gap distance between frame mounted poles and the coreof a rotating armature in electrical machinery.

BACKGROUND OF THE INVENTION

Air gaps in electrical machinery are required to be maintained withincertain dimensional tolerances for proper operation. The tolerances arechecked each time a rotor is changed, or when troubleshooting ortesting. If the measured air gap is not within the required dimensionaltolerance, then the air gap is adjusted to be within that range.Adjustment of the air gap to within the required dimensional tolerancehelps improve commutation, regulation, physical clearance, among othermachine parameters.

Contact methods such as a wedged feeler gauge type method havetraditionally been used to measure air gaps. According to this method, atapered bar or “wedge” (shown in FIG. 1 as 18) is coated with graphiteand then slid between the core and the pole until resistance is felt.The wedge is then removed, and the point where the core and the polescrape the graphite is measured. There are several problems with thismethod. The wedged feeler gauge type method is time consuming,cumbersome, and inconsistent. The method is prone to human error due tothe varying amounts of force a person can use to push the wedge in.Obviously, pushing the wedge 18 in more or less yields a different airgap measurement. The measurement difference is significant especiallysince the air gap distance is small and the measurements are taken inmicrometers. Additional variances in the measurement can result from theposition on the rotor from which measurements are taken. The position onthe rotor from which measurements are taken must be located under theexact center of the pole for an accurate minimum reading. Centering canbe difficult in areas with limited visibility, and the pole's centermust be aligned with a mirror. The method is also problematic becausethis method often requires the removal of internal bus work to getenough physical access to take the measurements. Unnecessary disassemblyadds additional time and risk during reassembly.

SUMMARY OF THE INVENTION

In one embodiment, a portable test kit for measuring an air gap betweena core and a pole of a rotor includes a capacitance sensor whichmeasures a minimum air gap between the capacitance sensor and a pole andwhich generates a voltage signal proportional to the measured air gap, apanel meter in communication with the capacitance sensor whichinterprets the voltage signal and displays the minimum air gapmeasurement between the core and the pole, an A/D converter incommunication with the capacitance sensor which converts the voltagesignal to a digitized signal, and a control panel which receives thedigitized signal from the A/D converter and processes the digitizedsignal. The control panel collects and displays the minimum air gapmeasurement, and displays a graphic of the core and surrounding poles.When testing between the core and one of the surrounding poles iscomplete, the graphic of the pole tested visually darkens to indicatethat portion of the test is complete. A case designed for portabilitycontains the capacitance sensor, the panel meter, the A/D converter andthe control panel.

In another embodiment, a non-contact test method for measuring an airgap between a rotor core and a stationary pole which includes the stepsof affixing a capacitance sensor to a core of a rotor. The capacitancesensor is arranged between the core and the stationary pole, capturingthe air gap measurement between the core and the pole with thecapacitance sensor, and displaying the air gap measurement.

In yet another embodiment, a non-contact test method for measuring anair gap between a rotating element and a stationary element, includesaffixing a capacitance sensor to a rotating element, the capacitancesensor being arranged between the rotating element and a stationaryelement, capturing an air gap measurement between the rotating elementand the stationary element with the capacitance sensor, and displayingthe air gap measurement.

In still another embodiment, a non-contact test method for measuring anair gap between a rotating surface and a stationary element includesaffixing a capacitance sensor to a stationary element, the capacitancesensor being arranged between the rotating surface and the stationaryelement, capturing an air gap measurement between the rotating surfaceand the stationary element with the capacitance sensor, and displayingthe air gap measurement.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a portion of a rotor core and a pole and shows awedged feeler gauge;

FIG. 2 illustrates a portion of a rotor core and a pole and shows theplacement of the sensor;

FIG. 3 illustrates the sensor offset;

FIG. 4 illustrates an embodiment of the test kit and shows the controlpanel;

FIG. 5 illustrates a calibration graph;

FIG. 6 illustrates a perspective view of a calibration block;

FIG. 7 illustrates a signal layout for the test kit;

FIG. 8 illustrates an enlarged view of the control panel shown in FIG.4; and

FIG. 9 illustrates a portion of a sensor attached to a fixed locationopposite a rotating surface.

DETAILED DESCRIPTION

A non-contact electrical machine air gap test kit for measuring the airgap distance between a stationary element such as frame mounted polesand a rotating element such as the core of a rotating armature inelectrical machinery includes a case containing a capacitance sensor, apanel meter, an A/D converter and a control panel. All of the componentsof the test kit including the sensor, the panel meter, the A/D converterand the control panel or panel PC are integrated together into a custompanel and case that is ideally configured for portability, such with ahandle 36 (shown in FIG. 4) or a size or a shape conducive toportability. The air gap being measured is the distance between the coreof the rotating armature and the frame mounted pole. As shown in FIG. 1,the air gap 12 is defined between a core 10 of a rotor and a pole 14mounted to a frame 16.

In the embodiment shown in FIG. 2, capacitance probe 20 a is incommunication with wire 22. The probe 20 a is secured to the core 10 andarranged to face the pole 14 or other target being measured. The probe20 a creates a capacitor between itself and the object being measured(pole 14). The controller 20 b (shown in FIG. 7) measures thiscapacitance. In the foreground of FIG. 2, a piece of tape 24 is shown.Tape 24 is used to provide strain relief and prevent rotational motionfrom pulling the sensor/magnet combination off of the core 10 of therotor. The capacitance probe 20 a is positioned between the core 10 andthe pole 14 within the air gap 12 defined between the core 10 and thepole 14. The capacitance probe 20 a is a modified commercial off theshelf (“COTS”) capacitive sensor and removably attaches to the core 10of the rotor beneath the stationary poles 14. In the embodiment of FIG.3, the capacitance probe 20 a is modified to include a magnet 26 and anexternal reference lead 24. The magnet 26 is used as a means forattaching the probe 20 a to the core 10. The magnet 26 is preferably aneodymium magnet. The magnet 26 is but one example of an adhesion deviceto attach the capacitance probe 20 a to the core 10. However, anysuitable means for attaching the capacitance probe 20 a to the core 10is acceptable and materials other than neodymium may be found suitable.

The capacitance probe 20 a is connected to the COTS controller 20 b(shown in FIG. 7) a fixture that is already referenced to ground. Thecontroller 20 b outputs 0-10 VDC proportional to 0 (inches) to FullScale (inches) of the probe, which in the example embodiment beingdiscussed would be 0.00″-0.200″, which is the distance from the face ofthe probe 20 a (which is at 0″) to the pole 14. The unit of measure inthis example is inches, but it could also be another unit of measuresuch as micrometers. Together, the probe 20 a and the controller 20 bcomprise the capacitance sensor 20.

The sensor 20 is configured to measure a minimum air gap between thecapacitance probe 20 a and pole 14, and configured to generate a voltagesignal proportional to the measured air gap 12. The measurement read bythe probe 20 a is a measurement reflective of the distance from the faceof the probe 20 a to the pole 14 but not the desired reading from thecore 10 to the pole 14. To account for this, the measurement needs tocorrected by the amount of an offset. The offset, shown in theembodiment of FIG. 3, is comprised of the thickness of the probe 20 aplus the thickness of the magnet 26.

The voltage signal output of the sensor 20 is interpreted and processedby panel meters 28, 30, shown in FIG. 4, and also by an analog todigital (“A/D”) converter 42. The A/D converter is not seen in FIG. 4,but is located internal to the panel PC 34 shown in FIG. 7. The panelmeters 28, 30 are in communication with the capacitance sensor 20 andconfigured to interpret the voltage signal. Panel meters 28, 30 displaythe minimum air gap for the particular pole 14 under test. An internalfeature of the meters 28, 30 is that they are configurable to displayand hold the minimum air gap reading between the core and the pole. Itis important to note that as the probe is rotated beneath a pole, thevalues measured go from maximum to minimum back to maximum. The minimumvalue is the value of interest. The minimum air gap measurement occurswhen the sensor is rotated under the center of the pole 14. The twopanel meters 28 and 30 are redundant. Generally the air gap readings aredone one at a time, however, with two panel meters 28, 30, the assemblycontains two channels. The spare channel provides a redundant channel inthe case of channel failure. The second channel also provides thecapability to measure both sides of the air gap 12 at the same time. Anair gap measurement at each end of the pole 14 checks parallelism of thepole face to the rotor core 10.

The sensor 20 is also in communication with the A/D converter which isin further communication with the control panel or panel PC 34. On theface of the control panel 34 is displayed a graphic of the core and itssurrounding poles. The face of the control panel or panel PC 34 is atouchscreen. An enlarged view of the face of the control panel is shownin FIG. 8. The control panel 34 is adapted to receive a digitized signalfrom the A/D converter and process the signal, and configured to collectand display the minimum air gap measurement.

The COTS capacitive sensor has a known linear accuracy. In a knowncapacitive sensor, the MTI Accumeasure 9000, the linear accuracy is+/−0.003 inches. Ideally, the accuracy of the device for thisapplication should be closer to +/−0.001 inches. The capacitive sensorcomes with a calibration report. An example of a calibration report isshown in FIG. 5. The report shows the percent full scale accuracy on they-axis versus the gap being read, on the x-axis. The sensor inaccuracyis a known constant. In order to improve the accuracy of the sensor, thecalibration report is used to create a linearizing correction curve.This is done by breaking down the curve in the calibration report intoten linear segments and creating a piece wise linear scale for each ofthe ten segments. Both the automatic and manual modes of the tool bothuse this piece wise scale method to correct for the know inaccuracy ofthe sensor.

With the help of a calibration block 40, shown in FIG. 6, systemaccuracy is verified prior to each measurement being taken. The sensoris placed within three gaps of a known dimension that are incorporatedon the face of the custom control panel, i.e., calibration block 40. Thegaps have three known steps spanning from the minimum dimension of theoffset, which in the exemplary embodiment is approximately 0.135 inches,to a maximum dimension of the Full Scale of the probe range, which inthe exemplary embodiment is approximately 0.335 inches. Using the piecewise correction method discussed above, the device measures within+/−0.001 inches; this is the acceptable tolerance for this measurementsystem and by verifying it prior to taking each measurement, theoperator is assured that the system is operating properly.

There are two operating modes, manual and automatic. The collected datais displayed through either the manual or automatic mode. The two modesmay be operated at the same time or independently. The standardoperating mode is considered to be the automatic collection mode. Ineither operation mode, the processing is common to both. The minimumdistance value between the rotor 10 and the pole 14 is considered the“air gap” and this is the distance displayed.

The manual mode makes use of the panel meters 28, 30 shown in FIG. 4.The panel meters 28, 30 yield a redundant display of the air gapmeasurements in real time. Redundant meters and modes allow for deviceoperation in the case of a automatic mode failure. In the manual mode,the capacitance probe 20 a which is secured to the core 10 is manuallyrotated under each pole 14 being tested to measure the air gap betweenthe capacitance probe 20 a and the pole 14, and the lowest minimum airgap reading is displayed on the meters 28, 30. Each pole has its ownminimum reading and the process must be repeated for each pole. Themeter is then reset manually and the process repeated for each of the 16poles 14 that are arranged about the core 10 in a 360° circle.

The automatic mode works in parallel with the meters 28, 30. Theautomatic mode operates with custom software built into the controlpanel or panel PC 34 and allows for automatic readings of all 16 airgaps through a 360° rotation. In the automatic mode, the capacitanceprobe 20 a secured to the core 10 is slowly advanced through a full 360degree rotation so that each of the 16 poles that surround the core maybe tested. As the capacitance probe 20 a is advanced, the customsoftware collects the minimum air gap reading for each pole. The controlpanel 34 collects and displays the minimum air gap measurement collectedfrom a set of measurements taken. Preferably three measurements aretaken at each station, and then from that grouping of measurements, theminimum air gap measurement is determined and selected to be displayedon the control panel 34 by the custom software. On the face of thecontrol panel 34 is displayed a graphic of the core 10 and itssurrounding poles 14, as shown in FIG. 8. When a test between the core10 and a pole 14 is complete, the graphic of the pole 14 tested visuallydarkens to indicate that the pole 14 has been tested and that portion ofthe test is complete. Because the sensor 20 measures a probe-to-targetdistance, an offset equal to the thickness of the probe plus thethickness of the magnet is added to the reading from the capacitiveprobe to determine the core 10 to pole (target) 14 distance. In theexemplary embodiment, an offset of 0.135 inches would be added to the0.000″-0.200″ reading from the capacitive probe to determine the core 10to pole 14 distance, 0.135″ being the thickness of the capacitive probe.

The air gap test equipment signal layout is shown in FIG. 7. Inoperation, the capacitance probe 20 a, which is the PCR-50 probe forexample, senses the air gap, and sends an electrical signal, in thisinstance a 16 kHz signal to the controller 20 b. The controller 20 b isthe MTI Accumeasure 9000, for example. The sensor 20 sends an output0-10 VDC which is read by the A/D converter and by the panel meters 28,30. The sensor 20 outputs a voltage >10 VDC when the pole is out ofrange. The >10 VDC correlates to >Full Scale inches, which indicates thetarget or pole is out of range. In the exemplary embodiment, the outputvoltage of >10 VDC correlates to >0.335 inches. The A/D converter mayfor example be the USP-6008 produced by National Instruments. The sensoroutput of the sensor 20 is a linear voltage proportional to the air gap.The A/D converter sends the sensor 20 output in a digital format ordigitized form to the custom software onboard the control panel 34 thatprocesses the data and produces the air gap measurement.

The custom software may be written for example with the Lab VIEWcompiler. The program works to allow for automatic readings of all 16air gaps through a 360° rotation. The program corrects to thecalibration curve, maintains each pole's profile and logs the minimumreading, accomplished through three major execution steps: 1) a datasurveying “while loop”; 2) a data collection “while loop”; and 3) anindicating “for loop.” A status flag is used to control process flow. Afalse status flag keeps the program in the data surveying loop. The datasurveying loop can change the flag value from false to true. A true flagtransfers the program to the data collection loop. At completion of thedata collection, the false status flag is reset to true.

The data surveying loop is the loop that initializes on program startupand initiates program flow. It is the overall while loop and iscontrolled by a “stop” button on the graphical user interface (“GUI”).Each iteration of the loop collects a distance value from the A/Dconverter. The expected values are from 0-Full Scale, which in theexample embodiment is 0.135″-0.335″. If the measured value is greaterthan Full Scale or in this example embodiment greater than 0.333″, it isassumed that the sensor is in-between two poles rather than under apole, and it checks for the distance value again. Data values greaterthan Full Scale is not processed and the status flag remains false. Ifthe first value measured is less than Full Scale, it is assumed that thesensor has been rotated beneath a pole. The data surveying loop in thiscase changes the status flag to true thereby enabling the datacollection loop.

The data collection loop is nested internal to the data surveying “whileloop” in the custom program. The status flag initiates the datacollection loop and interrupts the data surveying loop. The datasurveying loop is a while loop based on the current value. The expectedvalues in this loop are less than Full Scale. If the values are lessthan Full Scale, the data collection loop stays locked in. The currentvalue is added to an auto indexing array. This array continues to buildas long as the current value is less than Full Scale. If the first valueis greater than Full Scale then the data collection loop stops and thearray is complete. This would happen as the sensor 20 a is rotated pastthe pole being tested and is between two poles, just before comingbeneath the next pole to be tested. The array with the collected is thenprocessed to determine the lowest value. The lowest value is then sentto the indicating for loop.

The indicating for loop is internal to the data surveying loop of thecustom program and operates once when the status flag has been set andafter the data collection loop is complete. It displays the processedlowest value in the appropriate indication on the graphical userinterface or GUI. The initial display index is zero. The first time thestatus flag is set the lowest value is displayed at indicator “zero”.The status flag is reset to false and the display index is incrementedby one. Once the value is displayed the data surveying loop continues.The next time the for loop is called, the air gap measurement will bedisplayed in indicator “one”. The correlation between display index andthe actual pole being read relies on the user. For example index zero isalways the first pole measured, followed by the second pole measured.The custom program assumes the sensor 20 a is started in a particularlocation and rotated in one direction. In this manner, all 16 polemeasurements are successively displayed without interruption.Measurement of the entire machine takes less than five minutes. Postcorrection of the data, all readings are accurate within 0.002″. At thecompletion of the measurements a screen shot is logged for futureaccess, as shown in FIG. 8.

The uses of this kit are not limited to air gap testing but may includeother electrical motor diagnostic features. As shown in FIG. 9, one suchembodiment would include mounting the sensor 20 a to a fixed location 44and rotating a surface 46 beneath. The A/D converter combined with thepanel PC could monitor dynamic run out of surfaces such as slip ringswhile a machine is operational.

The many features and advantages of the invention are apparent from thedetailed specification, and thus, it is intended by the appended claimsto cover all such features and advantages of the invention which fallwithin the true spirit and scope of the invention. Further, sincenumerous modifications and variations will readily occur to thoseskilled in the art, it is not desired to limit the invention to theexact construction and operation illustrated and described, andaccordingly, all suitable modifications and equivalents may be resortedto, falling within the scope of the invention.

What is claimed is:
 1. A portable test kit for measuring an air gapbetween a core of a rotor and a pole of a rotor, comprising; acapacitance sensor configured to measure a minimum air gap distancebetween the capacitance sensor and the pole and configured to generate avoltage signal proportional to the minimum air gap distance; a panelmeter in communication with the capacitance sensor configured tointerpret the voltage signal and display a minimum air gap measurementbetween the core and the pole, wherein the panel meter is configured todisplay and hold the minimum air gap measurement; an A/D converter incommunication with the capacitance sensor configured to convert thevoltage signal to a digitized signal; a control panel adapted to receivethe digitized signal from the A/D converter and process the digitizedsignal, wherein the control panel displays the minimum air gapmeasurement, wherein the control panel displays a graphic ofsurrounding, and wherein the control panel processes the digitizedsignal including correcting the minimum air gap measurement with apiecewise linear scale created from a calibration curve associated withthe capacitance sensor; a calibration block, having known steps spanningfrom a minimum dimension of an offset to a maximum dimension of a fullscale of the sensor range, to receive the capacitance sensor wherein theoffset is equal to a thickness of the sensor plus a thickness of amagnet; and a case configured for portability containing the capacitancesensor, the panel meter, the A/D converter and the control panel.
 2. Theportable test kit according to claim 1, wherein the minimum air gap ismeasured between the core and one of the surrounding poles, a graphicdarkens to indicate that the pole is tested.